Field effect controllable semiconductor components, such as vertical power MOS field effect transistors for example, are often used for switching currents or for applying voltages to loads.
Such semiconductor components are provided with a semiconductor body equipped with a first and a second doped terminal zone. A channel zone doped complementarily with respect to the doping of the terminal zones is formed between the two terminal zones. Arranged adjacent to the channel zone is a control electrode, which is electrically insulated from the semiconductor body by means of an insulating material.
In the case of a MOS field effect transistor, the first terminal zone is referred to as the source zone, the second terminal zone is referred to as the drain zone and the control electrode is referred to as the gate electrode. In practice, the MOS field effect transistor is often constructed vertically, the source zone doped with a dopant of a first conduction type being formed at a first main surface of the semiconductor body and the drain zone likewise doped with a dopant of the first conduction type being formed at a second main surface opposite to the first main surface, and the channel zone heavily doped with a dopant of the second conduction type being formed at a second main surface opposite to the first main surface, and the channel zone heavily doped with a dopant of the second conduction type being formed between said two terminal zones.
A more weakly doped drift zone of the first conduction type is usually provided between the channel zone and the drain zone, the doping of said drift zone generally being predetermined by the doping of the semiconductor body. In the case of a vertical construction of the MOS field effect transistor, the gate electrodes are usually accommodated in trenches extending from the source zone, through the channel zone, right into the drift zone.
Against the background of continually advancing miniaturization and increasing efficiency of power electronic systems, MOS field effect transistors are intended to have a lowest possible on resistance Ron, on the one hand, and a highest possible breakdown voltage, on the other hand. A reduction of the on resistance can be achieved here by increasing the doping concentration in the semiconductor body, but this measure also has the consequence of decreasing the breakdown voltage in an undesirable manner.
A reduction of the on resistance without adversely influencing the breakdown voltage is achieved in the case of MOS field effect transistors with charge compensation. In the case of such semiconductor components, so-called compensation zones are incorporated in the semiconductor body, in particular in the drift zone thereof, said compensation zones being equipped with a complementary doping with respect to the doping of the drift zone. The semiconductor body can be doped more highly on account of such compensation zones, thereby significantly reducing the on resistance of the semiconductor component. However, if a reverse voltage is applied between the two terminal zones, a space charge zone propagates in the semiconductor body and, upon reaching the compensation zones, has the effect that the charge carriers of different conduction types from the compensation zones and the drift zone are mutually compensated for, so that the number of charge carriers is reduced, and a high breakdown voltage can be realized. Charge-compensated semiconductor components of this type are sufficiently known and described for example in DE 43 097 64 C2.
Independently of the presence of compensation zones, a parasitic bipolar transistor is formed by the sequence of differently doped zones in the semiconductor component, namely the two terminal zones with charge carriers of the same conduction type and the channel zone—arranged between said terminal zones—with charge carriers of the other conduction type. In this case, the channel zone forms the base of the parasitic bipolar transistor, while the two terminal zones form the emitter and collector thereof.
It has been shown, then, that during the operation of the semiconductor component, i.e. when a forward voltage is applied between the terminal zones of the semiconductor component and a drive potential is applied to the control electrode, charge carriers of the same conduction type accumulate in the channel zone, which may activate the parasitic bipolar transistor and thereby decrease the dielectric strength of the semiconductor component in an undesirable manner. In order to avoid such a reduction of the dielectric strength of the semiconductor component due to an activation of the parasitic bipolar transistor, special precautions have to be taken, for which purpose the source zone and the channel zone are usually short-circuited in the case of a MOS field effect transistor, which has the effect that these two zones are always at the same potential, so that no charge carriers can accumulate in the channel zone and activation of the parasitic bipolar transistor is prevented.
However, short-circuiting the source and channel zones has the disadvantage that this gives rise to a diode which, if no drive potential is present at the control electrode, can turn off only in one direction. This direction is usually referred to as the “forward direction”, in which case, for example with n-doped terminal zones and a p-doped channel zone, the diode turns off only when the drain electrode has a higher potential than the source electrode. If the forward voltage at the electrodes of the semiconductor component is subjected to polarity reversal, i.e. in the case of a forward voltage applied in the source-drain direction, the npn junction formed from terminal zones and channel zone with short-circuited source and channel zones conducts like a diode.
For many applications, in particular for the case where inductive loads are to be switched, it would be extremely desirable, however, to have available a field effect controllable semiconductor component which can turn off both in the forward direction and in the reverse direction if no drive potential is present at the control electrode. This prevents charge carriers from flowing into the semiconductor body in the case of a forward voltage biased in the reverse direction, said charge carriers leading to an undesirable initial voltage or current pulse in the event of a polarity reversal of the forward voltage in the forward direction.
The prior art has already specified solutions for achieving this object. Thus, EP 0 606 661 B1 proposes, for this purpose, canceling the short-circuit connection between the source zone and the channel zone and arranging instead a conductive connection to a resistor, thereby increasing the voltage drop upon application of a forward voltage in the reverse direction.
Furthermore, DE 100 60 428 A1 proposes, for this purpose, with source and channel zones not being short-circuited, the formation of a compensation zone in the channel zone and in the source zone, which has a material that promotes the recombination of charge carriers of the first and second conduction types. This prevents charge carriers from accumulating in the channel zone as a result of recombination.
U.S. Pat. No. 6,271,562 B1 describes a field effect controlled power semiconductor device having a low on resistance. In this device, source zones and base zones are connected to a source electrode via a contact hole in a known manner.
The U.S. patent application US 2001/0041400 A1 describes a method for the implementation of trench walls, in which the implantation beam assumes a slight angle with respect to the axis of the trenches.
U.S. Pat. No. 6,468,847 B1 describes a method for fabricating a high-voltage transistor, in which a body zone is not short-circuited with a source metallization.
The German patent DE 43 09 764 C2 describes a power MOSFET with compensation zones.
The published German patent application DE 102 26 664 A1 describes a semiconductor component with compensation zones, in which source and channel zones are short-circuited with one another.
The U.S. patent application US 2003/0181010 A1 describes a power semiconductor component with compensation zones, in which source and channel zones are short-circuited with one another.